The control of energy metabolism within intact tissues is being investigated using a variety of techniques and tissues. The relation between the rates of energy conversion, via mitochondrial oxidative phosphorylation and glycolysis, and work output is being measured in the heart in vivo, isolated perfused heart, isolated mitochondria and cultured cells lines. In all of these preparations a tight coupling between the rate of work and the rate of energy conversion has been observed. In order to gain insight into the mechanism of this coupling, several of the key metabolic intermediates are also being determined as a function of work output using non-invasive techniques. Adenosine di- and tri-phosphates, inorganic phosphate, creatine phosphate and pH are being monitored using 31P NMR. Mitochondrial NAD redox state is monitored using fluorescence spectroscopy. Classical models concerning the control of energy conversion within cells involve the intracellular concentrations of adenosine di- and tri-phosphates as well as inorganic phosphate. However, in our in vivo and perfused heart studies we have demonstrated that no change in intracellular adenosine phosphates or inorganic phosphate occurs with large changes in cardiac work output. Further, in both the isolated perfused heart and mitochondria studies we have demonstrated that the redox state of NADH can control the rate of mitochondrial respiration and that the NAD redox state does change appropriately (i.e. becomes more reduced) when the isolated perfused heart is stimulated to do more work. These data suggest that redox state of NAD may be a key intermediate in the coupling of work output with mitochondrial energy conversion in the heart.